1. Field of the Invention
The present invention relates to an optical space communication apparatus for performing communication by propagating an optical beam in a free space.
2. Related Background Art
FIG. 1 is a drawing to show the structure of a conventional optical space communication apparatus which can correct an offset of alignment of an optical axis for communication, in which there are lenses 1, 2 and a movable mirror 3 provided on an optical path for communication with a mate apparatus and there are a polarization beam splitter 4, a partial reflection mirror 5, a lens 6, and a light-receiving element 7 successively arranged on the reflection side of the movable mirror 3. The movable mirror 3 is engaged with a gimbal mechanism 8 so as to be rotatable as shown in the partial structural drawings of FIGS. 2A and 2B, and the gimbal mechanism 8 is engaged with a fixed member 9, such as a main body of a lens barrel, so as to be rotatable, whereby the movable mirror 3 can be rotated about orthogonal pitch axis and yaw axis.
Also, a light-emitting element 11 is provided through a lens 10 on the reflection side of the polarization beam splitter 4, and a position detector 13 such as a CCD or a segmental device is provided through a lens 12 on the reflection side of the partial reflection mirror 5. Further, an output of the position detector 13 is connected through a signal processing circuit 14 with a drive circuit 15, and an output of the drive circuit 15 is connected with an actuator 16 for driving to rotate the movable mirror 3 about the X-axis and with an actuator 17 for driving to rotate the gimbal mechanism 8 about the Y-axis.
For transmission in optical space communication, an optical beam emitted from the light-emitting element 11 is reflected by the movable mirror 3, while for reception an optical beam from the mate apparatus is guided via the movable mirror 3 onto the light-receiving element 7.
FIG. 3 shows a state in which an apparatus A happens to change its posture about the pitch axis because of an external factor such as vibration, impact, etc. For this reason, since a transmission beam from the apparatus A deviates, an apparatus B becomes incapable of receiving the beam signal.
In FIG. 4 which shows details of the apparatus A shown in FIG. 3, an optical path of a receiving beam Lr from the polarization beam splitter 4 to the light-receiving element 7 and the position detector 13 in a state in which the apparatus A does not happen to change its posture is depicted by dashed lines. As shown in FIG. 4, in the apparatus A the posture of which has been changed, the light spots respectively on the light-receiving element 7 and the position detector 13 move as depicted by solid lines. When the movement of the light spot is large such that the light spot goes beyond the light-receiving surface, the apparatus A becomes incapable of receiving signals.
In this case, a distance of movement of a light spot is first detected by the position detector 13, as shown in FIG. 4, and the change in posture of the apparatus is calculated in the signal processing circuit 14. Then the drive circuit 15 drives the actuator 16, based on the information, to rotate the movable mirror 3 in the direction of arrow J as shown in FIG. 5. By this, the position of the spot on the light-receiving element 7 can be kept at an initial position where the spot was located before the posture change, so that the transmission beam Lt can be directed toward the apparatus B, enabling to perform transmission and reception in communication with the apparatus B, as shown in FIG. 6.
In case of a change in posture about the yaw axis, the actuator 17 is driven to rotate the movable mirror 3 about the yaw axis, thus correcting an alignment offset.
In the conventional apparatus, dc motors or stepping motors are used as the actuators 16, 17.
The conventional apparatus, however, had the following problems. If the dc motors were used in the above-described conventional example, because the dc motors can supply only a small torque as compared with its large occupying volume, dissipation power would be increased and frequency response would be degraded. Further, an increased scale of the motors would result in increasing the size of the entire apparatus, increasing the weight, etc. In case of the stepping motors being employed, a problem is that alignment age accuracy is degraded because of the limit of angular resolution.
In addition, a problem of a drop in performance and reliability will arise because of abrasion of brushes in either case of the dc motors or the stepping motors.
Further, a drive force is transmitted to the mirror or the gimbal member through a rocking shaft supporting the mirror or the gimbal member in the conventional example. In that case, the diameter of the shaft is set relatively large in order to reinforce the rocking shaft and to realize joint structure with a motor shaft, resulting in necessitating a large bearing. This increases the frictional resistance, which degrades frequency characteristics.